1. Field of the Invention
The present invention relates to an initial correction factor operating device used for a tire pressure drop detecting device, for example, for finding an initial correction factor for eliminating the effect on the rotational velocities of the tires by a difference in effective rolling radius depending on an initial difference between tires.
2. Description of Related Art
In recent years, as a safety device of a four-wheel vehicle such as an automobile or a truck, devices for detecting the drop in air pressure of a tire (DWS) have been developed, and some of them have been put to practical use.
An example of a method of detecting the drop in air pressure of a tire is a method utilizing a difference among the respective rotational velocities F.sub.1, F.sub.2, F.sub.3, and F.sub.4 of four tires W.sub.1, W.sub.2, W.sub.3, and W.sub.4 mounted on a vehicle. This method utilizes the fact that the rotational velocity F.sub.i (i=1, 2, 3, 4) changes depending on the condition of the air pressure of the tire W.sub.i. That is, when the air pressure of any one of the tires W.sub.i drops, the effective rolling radius of the tire W.sub.i decreases. As a result, the rotational velocity F.sub.i of the tire W.sub.i increases. Therefore, the drop in the air pressure of the tire W.sub.i can be judged on the basis of the difference in the rotational velocity F.sub.i.
The effective rolling radius is a value obtained by dividing the travel distance of the tire W.sub.i in one free rotation in a loaded state by 2.pi..
A judged value used in detecting the drop in the air pressure of the tire W.sub.i on the basis of the difference in the rotational velocity F.sub.i is found, for example, by the following equation (1), for example (see Japanese Patent Laid-Open (KOKAI) No. 305011/1988, Japanese Patent Laid-Open (KOKAI) No. 212609/1992, etc.). ##EQU1##
If all the effective rolling radii of the tires W.sub.i are the same, the respective rotational velocities F.sub.i are the same (F.sub.1 =F.sub.2 =F.sub.3 =F.sub.4). In this case, the judging value D is zero. Therefore, thresholds D.sub.TH1 and D.sub.TH2 (where D.sub.TH1, D.sub.TH2 &gt;0) are set. When the condition indicated by the following expression (2) is satisfied, it is judging that there is a tire W.sub.i whose air pressure has dropped. When the condition is not satisfied, it is judged that all the tires W.sub.i have normal internal pressure. EQU D&lt;-D.sub.TH1 or D&gt;D.sub.TH2 ( 2)
The effective rolling radius of actual tires varies within a fabrication tolerance (such variation is referred to as an "an initial difference" hereinafter). That is, even if all of the four tires W.sub.i have normal internal pressure, the effective rolling radii of the four tires W.sub.i differ due to the initial difference. Correspondingly, the rotational velocities F.sub.i of the tires W.sub.i vary. As a result, the judging value D may be a value other than zero. Therefore, it may be erroneously detected that the air pressure has dropped, although it has not dropped. In order to detect the drop in the air pressure with high precision, therefore, it is necessary to eliminate the effect of the initial difference from the detected rotational velocity F.sub.i.